Heating elements used in heating devices for heating fluids can be permanently damaged if the heating device is run dry and if water boils away. Electrical resistance heaters based, among other things, on thick film technology are used as heating elements for water heating in household appliances, such as for example water boilers, coffee machines, dishwashers and washing machines. Specific safety requirements are imposed on these. For example, fused protection against fire, against leaks and against voltage breakdowns must be provided on the heating element. In particular, according to a valid standard, fused protection against voltage breakdown must be provided not via the existing building or house fuses. In order to meet this requirement, it is usual to use heat protection switches, pressure switches which detect the presence of a fluid or melting fuses which melt and separate the heating circuit in the event of the heating element running dry and the continuous heating thereof thus produced.
However, these fuses have considerable disadvantages. The afore-mentioned pressure switch has a relatively slow response behaviour. Melting fuses result in an irreversible separation of the heating circuit when triggered whereby the heating device exhibits a defect and thus results in a case for customer service, incurring high costs.
It would thus be desirable to have a fuse system which protects a heating device of the type specified previously simply, quickly and cost-effectively, independently of any appliance control system which may be present.
Known from EP 0 845 119 B1 is a circuit arrangement for regulating the temperature of a heating element whose resistance value is a function of temperature. For this purpose, the circuit arrangement has a current sensor means coupled to the heating element, which delivers an AC voltage at one output which depends on the alternating current flowing through the heating element. This signal and a reference value predefined by a user are supplied to a first detector means. This delivers an output signal if the temperature of the heating element exceeds a temperature set by the user. A switching means connected in series to the heating element is then controlled according to the pulse-pause principle by means of a switch control circuit.
A circuit arrangement for protecting a braking resistor in a traction motor is furthermore known from U.S. Pat. No. 4,035,692. This circuit arrangement has a power supply coupled to the braking resistor to deliver a current to the braking resistor by means of a switching means. A current sensor means coupled to the braking resistor at whose output a signal proportional to the current flowing through the braking resistor can be tapped and a first scaling means in the form of a potentiometer and a resistor connected in series thereto at whose output a signal proportional to the supply voltage of the braking resistor can be tapped, are supplied to a differential amplifier which delivers a voltage value corresponding to the temperature. This signal is used to protect the braking resistor from overheating by opening the switching means.